This invention relates to a flux-cored wire for electrogas arc welding. The invention relates, more particularly, to a flux-cored wire useful for electrogas arc welding such as automatic gas shielded vertical arc welding of austenitic stainless steel.
Heretofore, an arc welding technique using a sheathed metal electrode has been principally used in processes for the arc welding of stainless steel.
Gas shielded arc welding processes, such as argon shielded or CO.sub.2 gas shielded arc welding and non-shielded arc welding process, have recently been utilized broadly. A submerged arc welding process has been used for the welding of plates having more than 20 mm of thickness, but this known process requires large quantities of compositions and additional facilities. Moreover, since the process uses multi-layer welding, its working efficiency is low. Furtherore, the welded material has poor corrosion resistance in the welded portion, if the material is to be used as a corrosion-resistant material.
There are electroslag welding processes which involve a consumable nozzle electroslag welding using a solid wire, flux-cored wire and hoop wire, etc., and an electrogas arc welding process which uses principally flux-cored wire, etc., for automatic vertical welding. The electroslag welding process is mainly used for the welding of a steel plate, but if the width of penetration is broad, the welded portion is easily cracked and its notch toughness is low, these being distinct disadvantages. The electrogas welding process, as heretofore used broadly for the welding of reservoir tanks and the sideplates of vessels, uses large quantities of mild steel or high tensile steel, but there exists no wire for the exclusive use of the electrogas arc welding presently on the market, so far as applicants know, for stainless steel. In other words, major wires used for stainless steel which have presently come into the market are flux-cored wires and solid wires for the MIG welding process, solid wires for the submerged arc welding process, and flux-cored wires for the no-gas arc welding process. Consequently, it is impossible to apply these wires to the electrogas arc welding, due to its poor workability and performance including poor corrosion resistance. Accordingly, this process has not yet received any significant notice so far.
The basic difference between wire for electrogas arc welding and that for MIG welding resides in the content of flux in the wire. That is, the wire for MIG welding contains greater amount of flux than that for electrogas arc welding. In the case of MIG welding, slag is always accumulated above the bead to serve as a protection, as shown in FIG. 1. Since welding is always performed in the lateral direction on the new base metal, even if slag is formed in great quantity, the arc seldom sparks over the slag. Therefore, the wire for MIG welding can maintain a constant arc stability, weldability and workability (generation of spatter, fume, etc.) in MIG welding.
However, in the case of electrogas arc welding, as illustrated in FIGS. 2 and 3, the welding position is vertical and the metal is vertically deposited successively. Further, there is a certain limitation to the amount of slag which escapes in the direction of the copper shoes on both sides to protect the bead, as shown in FIG. 3. Therefore, if the wire for MIG welding is used for electrogas arc welding, extra slag which could not escape toward the copper shoes, cause the result that slag is excessively accumulated above the deposited metal, indicated by dotted lines. Under such situation, the electric arc comes to spark over the slag, resulting in impaired electro-conductivity and arc stability, and it may finally become impossible to generate the electric arc any more. Actually, when the arc sparks over the slag, workability is extremely worsened due to scattering of slag and spatter. If the arc is further given thereover, the wire fed is heated red hot and bent, and the wire feed is finally stopped by a short circuit.
If welding is further continued in spite of increased amount of slag, heat is taken away by such slag to cause imperfect penetration, slag inclusion and formation of many blowholes, resulting in a spoiled weld.
Flux-cored wire for no-gas arc welding contains even greater amount of flux than the wire for MIG welding, and it has already been experimentally ensured that such wire is quite useless for electrogas arc welding.
Use of solid wire for electrogas arc welding results in very poor weldability, and no desired effect can be obtained. As it is impossible to add arc stabilizer, slag forming agent and such to solid wire, welding by use of such solid wire invites enlarged particle size and increased amount of spatter, resulting in low deposition efficiency and poor economy. Also, as the bead is not protected with slag forming agent, there takes place vehement high-temperature oxidation, and Cr, Mn, Si and the like effuse in the form of oxides, so that the required deposited metal content is not obtained. Further, when the weld length increases, a large amount of spatter deposits on the nozzle, gas outlets of copper shoes, etc., to cause impediment to the welding operation. For these reasons, solid wire can not be immediately used for electrogas arc welding.
As already known, there are three structures, austenite, ferrite and martensite, representatively, of stainless steel. As to the weldability of these structures of the stainless steel, the ferrite form has a heat sensitivity so as to cause embrittleness with the result that weld cracks may easily occur, and the martensite form has still poorer weldability so as to cause weld cracks because of its hardenability. As a consequence, martensite requires an annealing after preheating and immediately after welding. The austenite form, on the other hand, has no disadvantages of hardenability and has low cracking sensitivity and is the most weldable of the three types, special types being excluded.